The present invention relates to a method of controlling a speed reduction ratio for a vehicle having a continuously variable speed transmission.
A controlling method of the speed reduction ratio for a vehicle provided with a continuously variable speed transmission, wherein a reference engine speed is predetermined in accordance with engine throttle opening, and then the speed reduction ratio is so controlled that the actual engine speed coincides with said reference engine speed, is known in the Japanese Patent laid-open publication No. 62(1987)-237164.
When a vehicle equipped with such a continuously variable speed transmission is decelerated by closing the accelerator opening with the accelerator pedal released, the reference engine speed corresponding to the accelerator opening is low, and therefore the engine brake will not function effectively. When it becomes necessary to decelerate the vehicle by releasing the accelerator pedal to close the accelerator opening and by applying vehicle brakes simultaneously, it is desirable to make use of the engine brake function effectively along with the vehicle brake to improve braking characteristics. However, the above-described speed reduction ratio control may not be able to utilize enough engine brake.
In many vehicles equipped with a continuously variable speed transmission, a shift lever is installed whereby a vehicle running speed is controlled by positioning it to either the "D" or "L" range. Ordinarily, in the "L" range is set a higher reference engine speed than in the "D" range, and in each range the speed reduction ratio is so controlled that the actual engine speed coincides with the reference speed.
When a vehicle is being decelerated by releasing the accelerator pedal and closing the accelerator opening (which corresponds to either the engine throttle opening or the accelerator pedal depression) to almost zero, the reference engine speed is set corresponding to such closed accelerator opening. The engine brake is more effective during running in the "L" range where higher reference engine speed is set than during running in the "D" range where a lower reference engine speed is set.
When decelerating a vehicle by removing a foot from the accelerator pedal to close the accelerator opening to almost zero and by applying the vehicle brakes at the same time, it is preferable to utilize the engine brake effectively, as mentioned previously, in combination with the vehicle brake to shorten a brake distance. With a vehicle running with the shift lever positioned at "D", however, the engine brake force is small as mentioned above when the vehicle is decelerated by releasing the accelerator pedal, and consequently the effect of the engine brake will not be effective in reducing the brake distance.
It is conceivable that, when a need arises to apply the vehicle brake while running in the D-range, the shift lever may be shifted to the L-range position by a driver to obtain a greater engine brake force, but such manual shifting often results in delayed timing, and may not practically make the engine brake force supplement the vehicle brake for an immediate deceleration.
As a ratio control method of the continuosly variable speed transmission whereby the vehicle is decelerated by taking advantage of the engine brake function, it is known to automatically reduce the speed reduction ratio when the engine brake functions. Also known is a method whereby the speed reduction ratio is so changed that the engine brake potential is varied in relation with the running speed of vehicle, as disclosed in the Japanese Patent Laid-open Publication No. 60(1985)-95263.
With such an automatic ratio control or a vehicle-speed-related ratio control, the engine brake potential is determined solely by the running state of the vehicle. Consequently, the driver may obtain only a uniform predetermined amount of engine brake force when decelerating from the same speed at which he runs the vehicle, irrespective of the driver's desire to decelerate the vehicle by utilizing stronger engine brake force. His only choice for further deceleration will be to depress more strongly the brake pedal to increase the effect of the vehicle brake. Thus, the controlling method of the engine brake force by means of a conventional ratio control can not provide any means of response that follows closely the desires of the driver.